What is the limit of psychological thoughts in Islam, and are fleeting thoughts intentionally entertained regarding creed considered apostasy or wickedness, if they are neither uttered nor acted upon, and the heart is reassured with sound faith?

It is out of God's mercy towards the Ummah that He forgives passing thoughts (hadith (definition) al-nafs) as long as they are not acted upon or spoken, because these thoughts come without a person's conscious choice. Therefore, they should be completely ignored and not dwelt upon, with the certainty that they do not harm or affect one's faith, and that Satan is the one who suggests that they are deliberate. The solution is to turn away from these thoughts and occupy oneself with what benefits in both religious and worldly matters.